1. Technical Field
The present disclosure relates an automatic frequency calibration circuit and an automatic frequency calibration method.
2. Description of Related Art
In many electronic systems, a frequency synthesizer is required to provide a stable and specific frequency. The frequency synthesizer can dynamically adjust an output frequency according to an actual requirement. FIG. 1 is a circuit schematic diagram illustrating a voltage-controlled oscillator (VCO) 100 of a conventional frequency synthesizer. A control voltage VTUNE can change a capacitance of a varactor 110 in a resonant cavity of the VCO 100. By adjusting the capacitance of the varactor 110, an output frequency Fosc of the VCO 100 can be changed.
FIG. 2A is a circuit schematic diagram illustrating the varactor 110 of the VCO 100. For a wideband system, a relatively large varactor is required to be used in the resonant cavity of the VCO 100, so as to achieve all of the target frequencies Fosc. However, the large varactor corresponds to an increased oscillator gain. FIG. 2B is a schematic diagram illustrating a relationship curve of the control voltages VTUNE and the output frequencies Fosc in the large varactor 110 of FIG. 2A. Utilization of the large varactor 110 can increase a slope KVCO (i.e. the oscillator gain) of the relationship curve of FIG. 2B, which may increase a phase noise of the VCO 100.
FIG. 3A is a circuit schematic diagram illustrating another varactor 110 of the VCO 100. To satisfy a requirement of the wideband system, and meanwhile reduce the slope KVCO (i.e. the oscillator gain), when the wideband VCO 100 is designed, the varactor 110 of FIG. 3A can use a small varactor 111 in collaboration with a switched-capacitor bank 112, so as to achieve an enough bandwidth without increasing the oscillator gain. Namely, the VCO 100 has a plurality of capacitor configurations. By changing connection states of one or a plurality of switches in the switched-capacitor bank 112, various possible values of the capacitance can be achieved.
FIG. 3B is a schematic diagram illustrating relationship curves of the control voltages VTUNE and the output frequencies Fosc in the varactor 110 of FIG. 3A. As shown in FIG. 3B, a single tuning curve with a large slope can be converted into a plurality of curves with small slopes. Each of the curves in FIG. 3B corresponds to one of the aforementioned capacitor configurations. Namely, when the VCO 100 is set to one of the capacitor configurations, the VCO 100 is operated according to one of the curves of FIG. 3B. An automatic frequency calibration circuit is used for finding the tuning curve (i.e. finding the corresponding capacitor configuration) covering a target frequency before the frequency synthesizer is locked at the target frequency.
When the VCO 100 is applied to an integer-N frequency synthesizer, since a division ratio of a frequency divider is a fixed value, the automatic frequency calibration circuit can directly compare a reference frequency REF and an output frequency DIV of the frequency divider according to such division ratio, so as to find a suitable tuning curve. However, when the VCO 100 is applied to a fractional-N frequency synthesizer, since the division ratio of the frequency divider is constantly varied to achieve that an average thereof is in line with a precise fraction, when the reference frequency REF and the output frequency DIV of the frequency divider is compared, the conventional automatic frequency calibration circuit may have errors in the comparison.
Most of the currently known automatic frequency calibration circuits are designed in allusion to the integer-N frequency synthesizer, though if such conventional automatic frequency calibration circuit is applied to the fractional-N frequency synthesizer, a problem is probably occurred. Assuming an integer part and a fractional part of a target division ratio in the frequency synthesizer are respectively K and M, and the frequency found by the automatic frequency calibration circuit according to the target division ratio is Fafc, since when the conventional automatic frequency calibration circuit finds the most suitable tuning curve, only the integer part is considered, the found Fafc is approximately K times of the reference frequency REF. However, after the calibration is completed, the locked frequency of the frequency synthesizer is K.M times of the reference frequency REF, so that there is 0.M times of error there between. Such error is probably as much as the reference frequency REF, which may lead to a result that the VCO 100 is locked at a control voltage with worse phase noise, or even cannot be locked.